Trans. Nonferrous Met. Soc. China 25(2015) 1286−1297 Characterization of an iron oxy/hydroxide (gossan type) bearing refractory gold and silver ore by diagnostic leaching Oktay CELEP, Vedat SERBEST Hydromet B&PM Group, Division of Mineral & Coal Processing, Department of Mining Engineering, Karadeniz Technical University, Trabzon 61080, Turkey Received 18 August 2014; accepted 10 December 2014 Abstract: A detailed characterization of an iron oxy/hydroxide (gossan type) bearing refractory gold/silver ore was performed with a new diagnostic approach for the development of a pretreatment process prior to cyanide leaching. Gold was observed to be present as native and electrum (6−24 µm in size) and associated with limonite, goethite and lepidocrocite within calcite and quartz matrix. Mineral liberation analysis (MLA) showed that electrum is found as free grains and in association with beudantite, limonite/goethite and quartz. Silver was mainly present as acanthite (Ag2S) and electrum and as inclusions within beudantite phase in the ore. The cyanide leaching tests showed that the extractions of gold and silver from the ore (d80: 50 µm) were limited to 76% and 23%, respectively, over a leaching period of 24 h. Diagnostic leaching tests coupled with the detailed mineralogical analysis of the ore suggest that the refractory gold and silver are mainly associated within iron oxide mineral phases such as limonite/goethite and jarosite-beudantite, which can be decomposed in alkaline solutions. Based on these characterizations, alkaline pretreatment of ore in potassium hydroxide solution was performed prior to cyanidation, which improved significantly the extraction of silver and gold up to 87% Ag and 90% Au. These findings suggest that alkaline leaching can be used as a new diagnostic approach to characterize the refractoriness of iron oxy/hydroxide bearing gold/silver ore and as a pretreatment method to overcome the refractoriness. Key words: refractory gold and silver ore; characterization; diagnostic leaching; mineral liberation analysis (MLA); alkaline pretreatment in average) carriers in the gossan ores from Rio Tinto 1 Introduction (Spain) [11−14]. Gold and silver extractions from these types of ores are often limited to 70%−80% Au and Refractory gold and silver ores yield low gold and 40% Ag since jarosite-type phases are not readily silver extractions in cyanide leaching [1]. A suitable dissolved during the conventional cyanidation pre-treatment process such as roasting [2], pressure process [15]. oxidation [3], bio-oxidation [4], alkaline [5,6] or alkaline The diagnostic leaching procedure developed by sulfide leaching [7] and, to a limited extent, ultrafine LORENZEN [16] is a characterization method to grinding [8−10], is often required to overcome determine the reasons of refractory behavior of gold and refractoriness and render the encapsulated gold and silver silver ores. It involves cyanide leaching to extract accessible to the lixiviant action of cyanide and oxygen. unlocked gold or silver after each stage of acid treatment The gossan ores are intensely oxidized, weathered using a variety of mineral acids selected to decompose or decomposed rock, usually the upper and exposed part gold/silver-bearing target mineral(s) [17−19]. The of an ore deposit or mineral vein. The gossan (or iron decomposition of some mineral phases in different acid cap) ores mainly consist of quartz and iron oxides, leaching stages may be represented by the following chemical reactions [20−22]: limonite, goethite, and jarosites. It has been reported that the natural jarosite-type minerals such as beudantite CaCO3+2HCl→CaCl2+H2O+CO2 (1) [PbFe3(SO4)(AsO4)(OH)6], argentojarosite [AgFe3- MS+H2SO4→MSO4+H2S [M=Zn, Cu, Pb, etc.] (2) (SO4)2(OH)6] and the silver-bearing plumbojarosite 3+ + 2− [(Ag,Pb)Fe3(SO4)2(OH)6] are the major silver (230 g/t Ag 2FeS2+10HNO3→2Fe +2H +4SO4 +10NO+4H2O (3) Foundation item: Project (8300) supported by the Research Foundation of Karadeniz Technical University, Turkey Corresponding author: Oktay CELEP; Fax: +90-462-3257405; E-mail: [email protected] DOI: 10.1016/S1003-6326(15)63725-6 Oktay CELEP, et al/Trans. Nonferrous Met. Soc. China 25(2015) 1286−1297 1287 SiO +6HF→H SiF +2H O (4) and silver-bearing gossan type ore was performed to 2 2 6 2 LORENZEN and van DEVENTER [18] used provide an insight into the refractory behaviour of the ore. diagnostic leaching to identify the refractory nature of The mineral liberation analysis was undertaken to determine the occurrence and liberation of gold and gold ores (only 1.3%−2.4% leachable gold with direct cyanidation). It was found that approximately 95% of Au silver, and to identify refractory silver-bearing phases. A in ore was extracted by means of selective destruction of new, modified diagnostic approach was developed for gossan type ore so as to identify the refractions of gold-bearing minerals by nitric and sulphuric acids within diagnostic leaching procedure. TEAGUE et al [23] silver-bearing minerals as well as gold-bearing phases. studied diagnostic leaching of a refractory gold ore and a Based on the results of these characterizations, an alkaline pretreatment process was proposed and bulk flotation concentrates. HENLEY et al [24] studied the characterization of a refractory telluride ore by a demonstrated on the ore sample. two-stage diagnostic approach. CELEP et al [22] concluded that the refractoriness of a refractory gold ore 2 Experimental was associated with the dissemination and encapsulation of the very fine gold particles largely within carbonates, 2.1 Materials The ore sample (~150 kg) was obtained from oxides, sulfides and, to a small extent, silicates present in the ore matrix. Although the diagnostic leaching Bolkardağ–Niğde (Turkey) gold and silver deposit. It procedure includes different acid leaching stages and the was crushed to <1 mm in a jaw crusher and then ground in a rod mill. The chemical composition of the ore subsequent cyanidation, it is inefficient to decompose jarosite type minerals. In this work, an alkaline leaching sample (Table 1 and Table 2) was determined using was adopted as a diagnostic leaching stage for inductively coupled plasma-emission spectroscopy (ICP-ES) after four kinds of acid digestion and X-ray decomposition of jarosite type minerals. Detailed characterization such as mineral liberation fluoressans (XRF). The gold and silver contents of the size, mineral associations and modal mineralogical ore were determined by lead collection fire assay fusion−gravimetric finish method. The ore sample was analysis is pre-requisite for plant design and mineral processing operations [25]. The distribution of precious found to contain 8.4 g/t Au, 167.0 g/t Ag, 2.6% Pb, 4.8% metal, grain size and mode of occurrence (liberation, Zn and 1.6% As mass fraction. Quartz, goethite, calcite and dolomite were identified as the major mineral phases exposure, and mineral association) and their gangue components were quantitatively determined by means of by X-ray diffraction analysis (Fig. 1). The optical automated scanning electron microscopic techniques mineralogical analysis of the ore sample was also performed under an ore microscope (Leitz Wetzlar 1432) (QEMSCAN/MLA). The mineral liberation analysis (MLA) system consists of a specially developed software for which the polished sections of a number of package and a scanning electron microscopy (SEM) hand-picked pieces were prepared from the as-received ore. equipped with an energy dispersive spectrum (EDS) analyzer. The analysis was operated on particles mounted Table 1 Chemical analysis of ore sample (mass fraction, %) in 30 mm epoxy resin blocks to provide automated quantitative mineral liberation characterization [26]. The SiO2 Al2O3 Fe2O3 MgO CaO Na2OK2O system utilized stable back-scattered electron (BSE) 23.9 4.31 36.9 1.3 6.18 0.06 0.87 signals defined by its average atomic number to generate TiO2 P2O5 MnO Cr2O3 CTotal STotal LOI images of mineral grains from a scanning electron 0.17 0.02 1.44 0.15 1.72 0.55 15.3 microscope (SEM). Mineral identification was verified by using of a single X-ray analysis at a point within the Table 2 Element analysis of ore sample (mass fraction, %) particle. Parameters such as liberation, mineral grain size, Ag Au Ni Ba Sn mineral locking and mineral associations were then 1.67×10−2 8.4×10−4 5.16×10−2 1.97×10−2 1.72×10−2 measured by the system. In this system, X-ray mapping was also used when similar BSE brightness of minerals Cd Sb Cu Pb Zn As was identified [27]. There are seven different operating 3.92×10−2 2.63×10−2 0.15 2.60 4.80 1.59 modes available for the MLA system: standard BSE liberation analysis (BSE), extended BSE liberation 2.2 Methods analysis (XBSE), sparse phase liberation analysis (SPL), 2.2.1 Sample preparation particle X-ray mapping (PXMAP), selected particle The crushed ore sample was ground by a rod mill X-ray mapping (SXMAP), X-ray modal analysis for 3−50 min at 50% pulp density. Ultra-fine grinding of (XMOD), and rare phase search (RPS) [28]. the ore was carried out using 2 mm ceramic ball at 800 In this work, the detailed characterization of a gold- r/min stirring rate in a laboratory scale pin-type vertical 1288 Oktay CELEP, et al/Trans. Nonferrous Met. Soc. China 25(2015) 1286−1297 NH4OH, Na2CO3 and NaOH [20], which was used successfully in a pre-treatment method to obtain the improved gold and silver extractions from antimonial refractory gold and silver ore by cyanidation [6]. In this work, potassium hydroxide was used to ensure the most effectively decomposition as alkaline digestion stage at diagnostic leaching procedure. Diagnostic leaching tests were carried out by acid/alkaline leaching stages (KOH, HCl, H2SO4, HNO3 and HF) to destroy specific minerals, followed by cyanidation of the residue from each stage to determine the amounts of gold, silver and other metal.